1. Field of the Invention
The present invention generally relates to a process for the preparation of alkoxysilanes, most preferably trialkoxysilanes, utilizing microwave or RF energy. More particularly, the present invention relates to a process for the preparation of alkoxysilanes wherein silicon metal and a copper catalyst are exposed to microwave or RF radiation in the presence of an appropriate alcohol and a catalyst to yield the corresponding trialkoxysilane.
2. Description of the Prior Art
Alkoxysilanes, specifically trialkoxysilanes, and particularly trimethoxysilane and triethoxysilane are important items of commerce. The alkoxysilanes serve as raw materials in the production of coupling agents that are critical to many industrial segments including adhesive and sealants, coatings, plastics, fabrics, medical devices, cosmetics and others. Although there are many reports of methods to produce trialkoxysilanes none provide a simple and sustainable process.
Alkoxysilanes are well known in industry and are used in the preparation of organosilanes that are suitable for use in many applications including their use as coupling agents.
Traditional chemical approaches to alkoxysilanes include the hydrochlorination of silicon metal with subsequent hydrosilylation and esterification. The first step of this three step process requires a significant equipment investment due to the high temperatures required and corrosion which results.
A direct two step method was subsequently developed which involved the direct reaction of alcohols with silicone metal. This process was limited to methanol as alcohols of higher chain length proved to be unreactive or to have little reactivity towards silicon metal under the process conditions. The use of methanol, while useful in creating a trialkoxysilane which can be further reacted, is limited by the elimination of the toxic methanol as a byproduct upon cure or by reaction with moisture.
Improvements in the direct chemistry approach that allowed preparation with higher chain alcohols utilized a heat transfer agent, such as, a solvent, temperatures of 230° C. to 240° C., and very long reaction times. The catalyst choice is reported to be critical as the catalyst must have solubility in the solvent. Copper II salts of carboxylic acids were found to perform effectively. Both the direct process and the hydrochlorination process are reported to be used within the industry. Fluidized bed processes are also reported, however, they are reported to suffer from hot spots and a significant reduction in selectivity.
U.S. Pat. No. 3,071,700 describes a process for the production of alkoxysilanes where finely divided silicon is reacted in the liquid phase by contact with alcohols and phenols yielding a mono-, di-, tri- and tetramethoxysilanes.
The direct synthesis of trialkoxysilanes is disclosed in U.S. Pat. No. 3,775,077 by Rochow. The patent teaches the preparation of trialkoxysilanes by directly reacting a copper-silicon mass suspended in a silicone oil with an alcohol at 250° C.-300° C. The copper-silicon mass contains about 10 weight percent copper and is prepared by heating the copper-silicon mass in excess of 1000° C. The method results in low yields of the trialkoxysilane.
U.S. Pat. No. 3,775,457 teaches the use of polyaromatic hydrocarbon oils as solvents for the direct process using finely divided silicon with cuprous chloride as catalyst. Although the cuprous chloride results in a yield improvement versus the activated copper-silicon mass of U.S. Pat. No. 3,775,077 the use of cuprous chloride results in the need for expensive corrosion resistant materials of construction for the reactor and related equipment. Additionally, the use of cuprous chloride acts to catalyze the reaction of the trialkoxysilane to the tetraalkoxysilane which reduces the yield of the trialkoxysilane.
Additionally, when methanol is a reactant for producing trimethoxysilane, the use of cuprous chloride leads to formation of HCl which will react with some of the methanol to yield methyl chloride and water. This result leads to inefficiency with regard to the methanol usage. Water produced by the reaction can react with the trialkoxysilane and tetraalkoxysilane to produce soluble and gelled siloxanes thus further reducing the efficiency of the reaction. The presence of water can also adversely affect the silicon conversion. Other patents, for example the Japanese Kokai Tokkyo Koho 55-28928, 55-28929, 55-76891, 57-108094, and 62-96433 which disclose the use of cuprous or cupric chloride are subject to the same limitations.
U.S. Pat. No. 4,727,173 discloses the use of copper (II) hydroxide as catalyst which avoids the limitations associated with cuprous chloride and provides high selectivity to the trialkoxysilanes. The preferred solvents are diphenyl ether, polyaromatic hydrocarbons and alkylated benzenes such as dodecylbenzene. However, when copper (II) hydroxide is used in combination with alkylated benzenes, such as dodecylbenzene, the direct synthesis of trialkoxysilanes becomes unstable after about 25-35 weight percent of the silicon has been reacted. When methanol is the alcohol reactant at temperatures above 220° C., the trimethoxysilane content declines after approximately 90-95 weight percent to approximately 50-60 weight percent and recovers again to 80-95 weight percent after approximately 60 percent silicon conversion. Coupled with the loss of selectivity is the enhanced formation of methane, water and dimethyl ether. Methane and dimethyl ether formation represent inefficient use of the alcohol reagent. The problems associated with water generation are noted above.
Alcohol dehydration and dehydrogenation are troublesome with the use of ethanol and other higher homologs in the direct synthesis approach. At some temperatures (>250° C.) alkenes, and aldehydes are formed in significant amounts at the expense of the desired trialkoxysilane. The presence of these undesired products can also have a negative effect on the catalytic activity in terms of inhibition. At lower temperatures (<220° C.) the alcohol decomposition products are less prevalent but the direct synthesis is impractically slow. Japanese Kokai Tokkyo Koho 55-2641 discloses the use of cyclic ethers to improve reactivity and selectivity to triethoxysilane when the direct synthesis is conducted in dodecylbenzene at these low temperatures. Cyclic ethers such as dibenzo-18-crown-6 are quite expensive. Others such as 12-crown-4 are toxic.
A process for producing controlled selectivity mixtures of trialkoxysilane and tetraalkoxysilane is described in U.S. Pat. No. 4,762,939. The use of a mixed solvent system is useful in controlling the selectivity between the tri- and tetra-substituted products over a wide range. An inert solvent along with a solvent that promotes the reactivity of the trialkoxysilane with alcohol to produce the tetraalkoxysilane represent the preferred mixture. The teachings are especially designed to produce the tetraalkoxysilane.
U.S. Pat. No. 4,762,938 describes the preparation of alkoxysilanes by reacting halosilanes with monhydric alcohols and a trialkyl phosphite. In reactions using chlorosilanes, the hydrogen chloride that is formed during the esterification must be removed quickly from the reaction mixture in order to ensure complete reaction, to obtain a hydrogen chloride free product and to prevent undesirable side reactions. To achieve this it is often necessary to utilized complex and expensive manufacturing processes and plants and is difficult to achieve in an industrial scale. In addition the resulting product mixture contains the strong smelling trialkyl phosphite and the close boiling points between the phosphite and products may make purification difficult.
A process for producing trialkoxysilanes including an activation step wherein elemental silicon and copper catalysts are activated, a reaction step wherein an alcohol is reacted with the activated silicon/catalyst complex and a purification step wherein a halide is introduced into the reaction mixture is described in U.S. Pat. No. 4,931,578. This process results in a more stable product. Although high levels of silicon conversion and high selectivity are reported, the activation sequence and overall time make this process unattractive.
U.S. Pat. No. 5,103,034 discloses a process to make alkyldialkoxysilanes and trialkoxysilanes using silicon metal and either an alcohol, an acetal and/or an orthocarboxylic acid ester. Over all conversions of silicon and the selectivity to the trialkoxysilane are both very low.
U.S. Pat. No. 5,527,937 discloses a process for the Direct Synthesis of triethoxysilane wherein copper chloride is the catalyst and tri- or tetra-toluenes and/or their alkyl substituted derivatives are the solvents and dimethylsilicone is used as an antifoaming agent. The polyphenyl solvents are expensive heat transfer agents.
U.S. Pat. No. 5,728,858 teaches the use of a reducing agent to improve the activity of the silicon-copper catalyst. Improved yields of the trialkoxysilanes are cited with the use of the alkylated benzene and polyaromatic solvents. Activation of the silicon-copper catalyst can generate impurities that can adversely effect the reaction so efforts must be taken to remove them prior to the reaction. Reaction times are also very long making the process impractical from a commercial perspective.
The use of surface active additives in the direct synthesis of trialkoxysilanes is described in U.S. Pat. No. 5,783,720. The additives which are described as silicone antifoaming compounds and fluorosilicone polymers are stated to shorten the reaction induction time and time to steady state rates. Extreme care must be taken to ensure the products are not contaminated with the surface-active agents and that such surface-active agents do not induce any adverse pathways in the reaction. Contamination of the products can cause severe application performance problems.
U.S. Pat. No. 6,242,628 describes the preparation of alkoxysilanes by reaction of a chlorosilane and an alcohol. The process as described is cited as producing low acidic chloride containing products and involves addition of a metal alcoholate to the separated product to neutralize the acid component followed by reduced pressure distillation. If the neutralization is carried out at relatively high temperatures the neutralization process gives rise to secondary reactions which result in a reduction of product yield. The presence of the acid chloride in the reaction mixture requires the use of expensive corrosion resistant equipment.
U.S. Pat. No. 6,380,414 describes a process wherein trialkoxysilanes are prepared by the reaction of silicon metal and an alcohol in the presence of copper (II) oxide. The use of copper (II) oxide is described to produce an alkoxysilane in high conversion from the silicon and with high selectivity with regard to trialkoxysilane to tetraalkoxysilane. The process is limiting as the copper (II) oxide is required to be of a very narrow particle size distribution and is preferably generated from freshly precipitated copper (II) oxide. Additionally, the time to reach conversion is very long requiring 22 to 28 hours.
JP-A-101168084 relates to the preparation of trialkoxysilanes by reacting silicon metal and alcohol over a copper (II) oxide catalyst which has water content of <3000 ppm. The low water content of the catalyst may require a thermal pretreatment of the catalyst and hence an additional reaction step.
EP-A 0 517 398 discloses a process for preparing trialkoxysilanes by reacting silicon with a solution of hydrogen fluoride or a salt which can be hydrolyzed to form hydrogen fluoride in a liquid primary or secondary alcohol, with or without the addition of a copper catalyst. However, the use of hydrogen fluoride is problematic, since hydrogen fluoride is extremely toxic and can attack glass. Furthermore, the actual reaction has to be preceded by a pretreatment step in this process since CuF2 itself is inactive as a catalyst.
The use of a copper salt as catalyst whose anion contains at least one non-hydrolyzable fluorine atom for preparing trialkoxysilanes is disclosed in U.S. Pat. No. 6,410,771.
The non-hydrolyzable fluorine containing catalyst can also be used with additional copper containing catalysts. Good conversion and selectivity are described, however, very long reaction times are required to complete the reaction.
U.S. Pat. No. 6,580,000 and U.S. Pat. No. 6,680,399 disclose the preparation of alkoxysilanes by reacting silicon metal with an alcohol in the presence of a cupric bis(diorganophosphate) catalyst. The reaction is carried out in a polymeric form of ethyl orthosilicate as solvent. The preferred catalyst is cupric bis(diethyl phosphate). The use of the cupric bis(diorganophosphate) while favoring the selectivity of the trialkoxysilane does not afford the selectivity associated with other catalysts. Additionally, the ethyl orthosilicate solvent contains 28% of one of the reaction products, tetraethoxysilane (TEOS). The presence of the product (TEOS) in the reaction mixture makes quantitative analysis difficult and shifts the equilibrium to the tetra substituted silicon. Very long reaction times are required to achieve the results disclosed.
Lipschutz, et al., in Organic Letters, 5(17), 3085-3088, describes the reduction of dialkyl ketones to trialkylsilyl ethers using copper hydride-ligand complex under classical synthetic conditions and with sodium tert-butoxide in the presence of microwave radiation. Microwave and RF irradiation can accelerate the rate of chemical reactions by way of localized superheating of the reaction mixture. In the presence of a metal catalyst this effect is further enhanced. In many cases, microwave and RF heating is more energy efficient than conventional heating methods. Silicon nitride and oxide films and nanowires have been prepared in a microwave plasma environment. Crystalline silicon nanoparticles have been produced by the microwave decomposition of silane. However, there are no reports of a process for preparation of alkoxysilanes which employs microwave or RF irradiation.
In view of the deficiencies of the known art, there remains a need for a simple direct synthesis process without the need for pre or post treatment, which can be completed in a reasonable period of time with high conversion and selectivity towards the trialkoxysilane. The use of controlled microwave or RF generator provides such a process directed to preparation of alkoxysilanes by applying microwave or RF irradiation to silicon and alcohol in the presence of a catalyst.